Angular inductive sensors have a structure similar to that of linear inductive sensors: they include a “transformer” fixed part with a fixed primary circuit and at least one fixed secondary circuit, and a movable part made up by a metal target which is rigidly connected to the mechanical part to be angularly monitored. The fixed primary circuit is generally formed from a coil or from a circuit printed on a flat surface.
A high-frequency alternating current flows in the primary circuit. This current produces a magnetic field at the same frequency as the current flowing in the primary circuit. Each secondary circuit is also fixed and is placed on the same surface while forming at least two loops. The successive loops have a substantially identical surface for the reasons indicated below. They are crossed and therefore have an opposite orientation (from a trigonometric perspective).
As a result of the couplings between the primary circuit and the loops having a same surface of each secondary circuit, the primary flux creates magnetic fluxes seen as being reversed from one loop to the other of each secondary circuit.
In general, the surface of the target is at least as large as that of a loop of the secondary circuit and the movable target movement then modifies the coupling between the primary circuit and each loop of each secondary circuit. The measurement of the voltage induced at the terminals of the secondary circuits therefore allows the position of the mechanical part to be known. Therefore the successive positions of the target in front of the loops of the secondary circuit produce, in the loops of each secondary circuit, a quantity of magnetic flux, and therefore a voltage. The development of this voltage varies according to the position of the target and includes relative increases and decreases. In the end, this variation produces a curve which is quite close to a sine curve.
In other words, a voltage is induced in the loops of each secondary circuit. The sign of this voltage depends on the direction of the loop. The algebraic sum of these voltages varies according to the movement of the target in front of these loops: centered on two loops, the target creates a zero-sum voltage. Since the surfaces of the loops are identical, the voltage has an absolute value maximum when the target of a sensor is facing each loop when the size of the target is substantially identical to that of each secondary circuit loop.
For an angular sensor, the target angularly covers each loop of the secondary circuit. For example, for a secondary circuit having a course equal to 360° (two loops of 180°), the angular opening of the target is 180°. For a secondary circuit having a course equal to 180° (four loops of 90°), the target is made up of two angular sectors with a 90° opening, the vertex of the angular sectors being opposite. Generally, the opening angle of the target is equal to half of the course of the secondary circuit.
The inductive sensors, in particular angular inductive sensors where the position measured for the target is an angle, have errors in the measurement of the target angular position, and therefore of the mechanical part to be monitored. Several solutions have been proposed to solve this problem.
Patent document WO 2005/098370, for example, describes the use of a reference secondary loop, this reference being used for a more accurate measurement of the target position, following a prior calibration operation. Moreover, the patent document FR 2964735 aims to reduce the measuring errors through the use of compensation loops placed on the secondary circuits, which allow some parasitic components of the alternating signal received at the terminals of the secondary circuits to be removed.
However despite these improvements, the known solutions do not provide the angular sensors with a sufficient linearity in order to obtain reliable results. Furthermore, the sensitivity of a sensor is characterized by this linearity defined by a fictitious straight line which would be that approaching, at best, the real relationship between the measured angular value and the real angular value of the position of the target over the entire measurement range. The sensitivity is then measured by the proximity between the real angular measurement of the sensor and this fictitious straight line.